Apparatus for development of electrostatic images



United States Patent [72] Inventors Roger A.Drex1er; FOREIGN PATENTSConrad AllmmmRwhwtenNew k 1,218,287 6/1966 Germany 118/637 1%; 1 gap; gzfi g 17,228 8/1965 Japan 118/637 1e e [45] Patented Dec. 1, 1970 OTHERERENCES [73] Assignee Easman Kodak Compmy Art1c l e by Medley, H. C.Development of Electrostatlc Rochester New York Images IBM TechmcalBulletln, Vol. 2, No. 2, (August I a corponfion New Jersey 1959) pp 4,5. Copy In 1 18-637 Primary Examiner-Morris Kaplan Attorneys-Robert W.Hampton and Leonard W. Treash, Jr. [54] APPARATUS FOR DEVELOPMENT OFELECTROSTATIC IMAGES 17 Claims 2 Drawing ABSTRACT: Developer is fed intoa cavlty faster than 1t 1s dlscharged creatlng a developer accumulat1on1n the cavlty [52] 118A537 forming a soft, agitated development medium.In preferred Bosh 5/02 embodiments, two magnetic brushes are arranged tocreate [50] Field ofSearch 118/637, he accumulation in the f f a gemlytumbling rollback 637(MCH0; 1 7/175; gslunqmred); from the contacting ofone of the brushes with the image-bear- 346/74(ES) ing surface. In thepreferred embodiments, the accumulation is accentuated by adustin the sacin between the brushes [56] References cued and the image-bearin gsurf ce, adgustin g the peripheral speed UNITED STATES PATENTS of thebrushes and controlling the direction of movement of 3,133,834 5/1964Sowiak 118/637 the image-bearing surface relative to the movement of the3,145,122 8/1964 Streich,Sr... 118/637 brushes. 3,219,014 1 1/1965 Mottet a1. 1. 1 18/637 Carrier carryout is reduced by directing a strongmagnetic 3,256,855 6/1966 Oliphant 118/637 flux at the rollback area andby providing an auxiliary carrier 3,355,288 11/1967 Matkan 117/175scavenging mechanism.

Patented. Dec. 1, 1970 ,54 0

51 FIGJ A 3 9 :5 H 2 I i f r \X I ROGER A. DREXLER CONRAD ALTMANNINVENTORS ATTORNEYS APPARATUS FOR DEVELOPMENT OF ELECTROSTATIC IMAGESCROSS- REFERENCES TO RELATED APPLICATIONS Reference is made to commonlyassigned cofiled U.S. application Ser. No. 709,280,now US. Pat. No.3,457,900, entitled Single Magnetic Brush Apparatus for Development ofElectrostatic Images filed even date herewith in the name of Roger A.Drexler.

BACKGROUND OF THE INVENTION This invention relates to electrostaticimages (which term is meant to include all electrostatic chargepatterns, regardless of the method of formation) and, more specifically,to methods and apparatus for developing electrostatic images carried onan insulating surface. 1

In electr'ography, it is common to form an electrostatic image on aninsulating surface and to develop that image by applying toner particlesthereto. The resulting toner image is then utilized. In the most commoncommercial applications, the toner is either transferred in imageconfiguration to another surface and then fixed or is fixed to theinsulating surface itself. In processes in which the toner istransferred from the insulating surface prior to fixing, the insulatingsurface generally is reused.

Triboelectric developing systems have been adapted to the generaldevelopment of electrostatic images. In such systems, finely dividedtoner particles are held to the surface of much larger carrier particlesby electrostatic charges created by triboelectrification, forming amixture (herein called a developer). When the developer is brought intocontact with an electrostatic image, the attraction of the image for thetriboelectrically charged toner overcomes the attraction of the carrierfor the toner and the image is developed.

Among triboelectric developing systems, the most commonly used arecascade'systems and magnetic brush systems. In cascade systems, gravityis used to roll developer, across the image. Because cascade systems usegravity as their primary moving force, they are necessarily speedlimited. In automatic machines, a cascade recirculation system generallyrequires substantial machinespace.

ln magnetic brush systems, the carrier particles are ferromagnetic innature. These ferromagnetic carrier particles are held to an applicatorsurface, for example, a nonmagnetic cylinder, in bristle formation bymagnetic attraction. With the proper use of applicator, one or moremagnets and developer, the bristles can be brushed across a surfacecarrying an electrostatic image. The electrostatic attraction of thetoner for the image overcomes the triboelectrically created attractionbetween toner and ferromagnetic particles and the image is developed.Areas of the image exerting less attractive force on the toner than isexerted by the carrier are cleaned of toner as they are brushed. This iscommonly aided by application of an electrical bias to the carrierthrough the applicating surface of the brush. Magnetic brushes also havebeen designed to give either fringe or solid area development byadjusting the conductivity of the carrier. They can also be made to toneareas of less charge and clean areas of greater charge giving what isknown in the art as a reverse development.

Commercial applications of magnetic brush development have been quiteextensive. However, such applications generally have been concentratedin the area of development of electrostatic images'formed on anonreusable insulating surface, such as zinc oxide coated paper. Thatis, the toner image is fixed to the insulating surface carrying theelectrostatic image rather than beingtransferred prior to fixing.

In applying magnetic brush development to systems in which the surfaceto be toned is to be reused after utilization of the toner pattern,certain problems are accentuated. For example, repeated rubbingby thefairly coarse bristles of the magnetic brush has a tendency both toscratch the insulating surface and to create a toner scum on it. Theseproblems are particularly severe when the insulating surface isphotoconductive, as in xerography, because of difficulties in properimage formation on a scratched or scummed photoconductive surface. 7

Additionally, a powder transfer process greatly increases imagedegradation caused by carrier carryout. More specifically, small carrierparticles are commonly picked up by the insulating surface in the toningoperation and have the effect at toner transfer of holding a large areaof the transfer surface away from the photosensitive surface therebyinhibiting powder transfer. This problem is much reduced if no transferof toner is included in'the process, as is the case when the powder isfixed to the toned surface.

- SUMMARY OF THE INVENTION It is an object of this invention toeffectively tone an electrostatic image.

It is another object of this invention to minimize surface scratchingand scumming resulting from magnetic brush development of a reusableinsulating surface.

It is another object of this invention to improve triboelectricdeveloper agitation in magnetic brush development thereby providing goodquality development with low toner concentrations.

It is another object of this invention to reduce carrier carryout inmagnetic brush development.

It is another object of this invention to effectively tone areas desiredto be solid black.

It is another object of this invention to provide a magnetic brushdeveloping apparatus with a soft, well-mixed toning medium and reducedcarrier carryout, which is particularly adapted for use with a reusablephotoconductive surface.

According to the invention, scratching and scumming are reduced by adeveloper feed and discharge arrangement which provides a substantialdeveloper accumulation in a cavity accessible to the image-carryingsurface. This accumulation gives a soft, less abrasive toning mediumwhich, in turn, gives effective toning, including toning of solid areas.

It is a feature of a preferred form of this invention that the feed anddischarge arrangement is so constructed to gently tumble the developeraccumulated in the cavity, giving thorough mixing and triboelectriccharging, and thereby providing effective toning at remarkably low ontoner concentrations.

Although the accumulation of toner can be created and agitated with anumber of forms of apparatus within the concept of this invention, it isa preferred feature of this invention to position two cylindricalmagnetic brushes so that portions of their outer surfaces form a cavitywith the insulating surface to be toned. One brush (herein called thefeed brush) is rotated rapidly to bring developer into the cavity andbarely contacts the image-bearing surface which is moving in theopposite direction. The other brush (herein called the discharge brush")is rotated at a slower speed in the same direction. It picks updeveloper from the feed brush and brushes it lightly across theinsulating surface while carrying the developer out of the cavity.Because of the position and rates of rotation of the two brushes, theform of the magnetic fields created by the-brushes, and/or the relativedirection of motion of the image-bearing surface, there is a buildup ofdeveloper in the cavity. This accumulation of developer is in the formofa rollback-from the discharge brush, although it may fill the cavity.Because this developer is not as closely associated with the magneticfields of the brushes as developer actually being carried by thebrushes, it is softer and not as bristly or coarse, giving less abrasionand scumming of the insulating surface. The motion of the brushes andthe imagebearing surface causes developer to tumble gently, givingthorough and constant mixing of toner and carrier and good triboelectriccharging.

With the lessening of the influence of the magnetic field, there may bea tendency toward more carrier carryout on the insulating surface. It isa feature of this invention to reduce good triboelectric charging andeffective mixing.

' this carrier or iron carryout by careful placement of a strong magnetin the feed brush so thatit creates a substantial ma'g- I neticattractive force directly in the rollback area.

It is also a feature to further reduce carryout, by the feed brushitself which touches the insulating surface so lightly or not at allthat it does very little toning, but is close enough to scavenge awayfrom the surface, carrier that may haveadhered'thereto despite theaction of the strong magnet.

It is also a' feature of this invention to do further carrier scavengingwith an auxiliary magnetic scavenging structure more fully describedbelow.

' Other objects and inventive features will appear from the.morespecific description of the preferred embodiments of the inventionbelow.

BRIEF. DESCRIPTION OF THE. DRAWINGS I FIG. 1 is a schematiccross-sectional view of a magnetic brush assembly constructed accordingto the invention.

FIG. 2 is a fragmentary top view showing the drivetrain of themagnetic'brushjassembly of FIG. 1. i

ISESICRIPTION"QFTHEPREFERRED EMBODIMENTS around which is mounted astationary magnetic pole piece 25 thatgmay be made of soft steel orother magnetic material. Mounted around part of. the circumference ofthe pole piece 25 is a series of permanent magnets 6, for example,rubberbonded barium ferrite magnetic strips or poles. Concentric withthe arrangement of theseelements and on'the outside thereof arerotatable, preferably surface-roughened, nonmagnetic rollers 7 and 8.Each brush is constructed so that, as the rollers 7 and'8 rotate,developer, particles are held on its surface and moved with the rollerwhile in thefield of the magnetic strips. The magnetic strips 6 arearranged so that a section of each roller is not in enough of theinfluence of the magnetic field to hold the developer on the roller.

As shown in FIG.. 1, the two magnetic'brushes ,4 and are Obviously,developer cannot build .up in the rollback cavity indefinitely. Excesstoner fed into the, brush after the rollback is built up overflows atthe ends of the cylinders and falls into the reservoir 11 to be reused.

Ferromagnetic carrier particles in the rollback area have a tendency tobe occasionallycarried away by the insulating surface. This can bereduc'edby aiming"'a strong magnet I9 in the second brush at therollback 9. The strength of this magnet is preferably worked outempirically for the softness desired in the rollback and the carryoutpermissible. Further, the feed brush, which lightly contacts theinsulating surface after the surface passes the rollback area, iseffective to remove some of those carrier particles actually picked upby the electrostatic image in toning.

A scavenging device 30 can also beused. It includes a stationary singlestrong magnet 31 placed in close proximity with theinsulating surface 2and a rotatable nonmagnetic roller 32 between the magnet and surface 2to carry out ferromagnetic particlesattracted toward the magnet.

Many alternative mechanisms may be used to drive the rollers 7 and 8 atdifferent speeds. One such mechanism is shown in FIG. 2 in which asingle drive gear 29'connected to a source ofpower, not shown, drives agear connected by a shaft 26 to roller 8 and drives a gear 27, which hasa larger diameter than gear 28, and is connected through a shaft 24 toroller 7. The difference in the diameters of the gears will cause roller8 to be rotated at a faster speed than roller 7. Alternatively, thegears can be made the same size and the rotations per minute .ofvtherollers 7 and 8 be made equal, but the second roller 8 be made of largerdiameter than the first roller 7, thereby provid ing a greaterperipheral speed. Alternatively, the size and speed of rotation of therollers can be made equal, but the strength of the magnets and, hence,the size of the bristles in the feed brush made greater than in thedischarge brush.

Similarly, many alternative means may be used for replenishing tonerused up in the process. For example, as shown in FIG, I, a tonerdispenser 50 including a hopper 51 allows toner to fall at a regularrate onto a paddle wheel 52 which rotates and feeds toner into thetrough 11. Fresh toner is circulated gradually toward the bottom of thefeed brush 5 by paddle wheels 21'and 22, where it is picked up by thesecond brush'to begin use in the process.

arrangedso that the feed brush 5. feeds developer to the t dischargebrush 4. If a close doctor blade is not used with the discharge brush todefine a short'bristle size, a rollback 9 of developer will occur at apoint just prior to contact of the discharge brush with the insulatingsurface. This rollback can be enhanced by several features shown in thedrawing. First, the brushes are rotated so that they move in adirectionopposite to the movement of the insulating surface 2 at the points ofcontact therewith. Second, the feed brush 5 is rotated at afaster ratethan the discharge brush 4. Third, the feed brushis spaced farther fromthe insulating surface 2 than the discharge brush. Either of the secondor third features, if accentuated, are sufficient alone to create anaccumulation of developer in cavity 15 and, hence, a usable rollback.How ever, a preferred form of the invention uses all three features incombination to create a rollback of developer of substantial size, whichmay substantially fill the cavity l5-between the brushes and theinsulating surface- With a substantial, accumulation ofdeveloper in theroll-.

back cavity .15, this arrangement does most of its effective toning inthevsofter rollbackarea 9 with very, little toning clone in areas 12 and13 of stiffer bristle. The developer in the rollback which contactsthesu'rface2 is not held as firmly by'the magnetic fieldsof either brush asthe developer in bristle formation on the rollers and is therefore lessabrasive'to the surface 2. In addition, it is constantly being gentlytumbled, giving In addition to producing a softer medium which is lessharmful to a photoconductive or other insulating surface, the

broad soft development surface also gives excellent image resolution.The rollback action provides a continuous soft mixing of toner andcarrier helping to maintain theproper polarity of charge on the toner atthe development area. This continuous mixing combined with theeffectiveness of the toning allows for substantial variations in overalltoning concentrations with toning results that are very close inquality. If the brushes are used as development electrodes, theapparatus will give especially uniform blacks in solid image areas.

As an example of the invention, two brushes are constructed as shown inFIG. 1. The rotating cylinders 7 and 8 are made 3 inches in diameter andone-fourth inch apart at their closest outside points. The strip magnets6 have enough magnetic attraction for iron particles of +80- 120 mesh toproduce bristles approximately one-fourth inches long. The axes ofcylinders 7 and 8 are offset with respect to the insulating surface 2 byone-sixteenth inch with the periphery of feed cylinder 8 approximatelythree-sixteenths inch and "discharge cylinder 7-approximately one-eighthinch from insulating surface 2, respectively, at their closest points.With surface 2 moving at 20 inches a second, feed cylinder 8 is rotatedat 65 r.p.m. and discharge'cylinder 7 is rotated at 40 r.p.m. Excellenttoning is obtained at these speeds with bias adjustments as in priormagnetic brushes. As toner concentration is varied from 3 to 6 percentby weight, no difference in print quality is noticeable to the nakedeye. This remarkable and highly useful feature is believed to be due tothe excellent mixing and triboelectric charging obtained as thedeveloper tumbles in the rollback cavity 15. Although 65 and 40 r.p.m.appear to be optimum speeds for these parameters, little differenceactually is noticed if the cylinders 8 and 7 are rotated at a variety ofother speeds, for example, at 40 and 30 r.p.m., respectively. Theparameters giving best results with any particular equipment, developerand type of image are preferably worked out empirically with thatequipment, developer and type of image.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effective within the spirit and scope of theinvention as described hereinabove and as defined in the appendedclaims. For example, although the invention has been described withreference to magnetic brush means for transporting toner into and out ofa rollback cavity, it can be seen by such description that other meansof creating a soft tumbling development medium can be used within thespirit of the invention. For example, fiber brushes, rollers withridges, etc., could also be used to create a similar effect.

We claim: 1. Developing apparatus for use in applying toner to anelectrostatic image carried on an insulating surface, said apparatuscomprising:

means defining a cavity adjacent to said insulating surface; developerfeed means for feeding developer into said cavity at a first rate;

developer discharge means for discharging developer from said cavity ata second rate which is less than said first rate; and

whereby a substantial quantity of developer accumulates in and iscontinuously circulated through said cavity in contact with saidinsulating surface.

2. Developing apparatus for use in applying toner to an electrostaticimage carried on a moving insulating surface, said apparatus comprising:

means defining a cavity adjacent to said insulating surface;

developer feed means for feeding developer into said cavity at a firstquantity per unit of time;

developer discharge means for discharging developer from said cavity ata second quantity per unit of time; and

said first quantity per unit of time being greater than said secondquantity per unit of time thereby causing a substantial accumulation offreely tumbling developer in said cavity in contact with said insulatingsurface.

3. Developing apparatus for applying toner to an electrostatic imagecarried on a moving insulating surface to form a toner image defined bysaid electrostatic image said apparatus comprising: I

means defining a cavity at a developing position;

first magnetic brush feed means for feeding developer into said cavityat a first rate; and

second magnetic brush means for discharging developer from said cavityat a second rate less than said first rate so as to create anaccumulation of developer in said cavity and tumble developeraccumulated in said cavity through a path touching said movinginsulating surface.

4. Developing apparatus for use in applying toner to an electrostaticimage carried on an-insulating surface, said apparatus comprising:

means for moving said insulating surface in a first direction;

a developer comprising a mixture of toner powder and carrier powder;

developer feed means and developer discharge means, portions of whichcombine with a portion of said insulating surface to define arollback-cavity;

said feed means including means to feed developer into said rollbackcavity in a direction generally opposite said first direction and at afirst rate, and said discharge means including means to dischargedeveloper from said rollback cavity at a second rate less than saidfirst rate; and

said feed means and discharge means including means cooperating withsaid insulating surface to tumble developer accumulated in said cavitythrough a path a portion of which contacts said insulating surface.

' 5. Developing apparatus for use in applying toner to an electrostaticimage carried on a moving insulating surface, said apparatus comprising:

means defining a cavity adjacent to said insulating surface;

magnetic brush means for feeding developer into said cavity at a firstquantity per unit of time; and

magnetic brush means for discharging developer from said cavity at asecond lesser, quantity per unit of time, whereby a substantialaccumulation of freely tumbling developer is provided in said cavity incontact with said insulating surface.

6. A developing apparatus for use in applying toner to an electrostaticimage carried on an insulating surface moving in a first direction, saidapparatus comprising:

means defining a cavity adjacent said insulating surface;

first magnetic brush means for feeding developer into said cavity at afirst rate;

second magnetic brush means for discharging developer from said cavityat a second rate which is less than the rate at which developer is fedinto said cavity by said first magnetic brush means; and

said first and second magnetic brush being adapted to feed developerthrough said cavity in a second direction opposite to said firstdirection and being located with respect to said moving insulatingsurface so as to provide a gently tumbling accumulation of developer insaid cavity.

'7. A developing apparatus for use in applying toner to an electrostaticimage carried on a moving insulating surface, said apparatus comprising:

a developer comprised of a mixture of toner powder and magneticallyattractable carrier powder;

reservoir means for holding said developer;

magnetic means for feeding developer from said reservoir means to arollback cavity, cavity, and by a portion of said magnetic means and aportion of said insulating surface,

and for discharging developer from said rollback cavity back to saidreservoir means, said magnetic means includa. feed magnetic brush meansfor moving developer from said reservoir into said rollback cavity; and

b. discharge magnetic brush means for moving developer from saidrollback cavity into substantial contact with said moving insulatingsurface and then into said reservorr;

means for operating said feed and discharge magnetic brush means to movedeveloper at predetermined rates, said predetermined rates being sochosen relative to the move- .ment of said insulating surface to createa substantial accumulation of developer in said rollback cavity.

8. Apparatus according to claim '7 wherein at least a portion of saidmagnetic means is adapted to cooperate with said moving insulatingsurface to form a means for tumbling said developer accumulated in saidrollback cavity.

9. Apparatus according to claim 7 wherein said means for operating saidfeed and discharge magnetic brush means is adapted to move the developerbeing moved by said feed brush faster than the developer being moved bysaid discharge brush, thereby contributing to said accumulation ofdeveloper.

10. Apparatus according to claim '7 wherein said discharge brush meansis adapted to move developer in a direction opposite to the movement ofsaid insulating surface, thereby creating a rollback of developer.

11. Apparatus according to claim 7 wherein a portion of said feedmagnetic brush means is positioned close enough to said insulatingsurface to remove ferromagnetic particles deposited on said surface bysaid discharge brush means and by said developer accumulated in saidcavity.

12. Apparatus according to claim 10 wherein said feed magnetic brushmeans includes a strong magnetic flux producing means whose lines offorce extend substantially into said rollback of developer.

13. Apparatus according to claim 7 including an auxiliary magnetic meanspositioned subsequent to said magnetic means in thepath of saidinsulating surface for removing ter- 4 rornagnetic particles depositedon said insulating surface by said magnetic means and by said developeraccumulated in said cavity. g

14. Electrographic developing apparatus for use in applying toner to anelectrostatic charge pattern carried on a'moving insulating surface toform a toner pattern defined by said charge pattern, said apparatuscomprising:

a reservoir for holding magnetic brush developer;

magnetic means for moving developer from said reservoir to a rollbackcavity defined by portions of said magnetic means and said insulatingsurface and from said rollback cavity back to said reservoir, said meansincluding:

a. feed magnetic brush means for moving developer from said reservoirinto close proximity with said moving insulating surface and then intosaid rollback cavity; and

b. discharge magnetic brush means for moving developer from saidrollback cavity into substantial contact with said moving insulatingsurface and then into said reservoir, said feed and discharge magneticbrush means each having:

1. magnetic flux producing means, and v 2. a rotatable nonmagneticcylinder, a portion of which cylinder is positioned within the influenceof said magnetic flux producing means, said flux producing means beingpositioned to attract developer to the outer surface of said cylinderportion, said magnetic flux producing means being arranged in said feedmagnetic brush means and said discharge magnetic brush means so thatdeveloper in said reservoir is attracted by the magnetic flux producingmeans of said feed magnetic brush means and held on its correspondingcylinder when rotated until said developer comes under the attraction ofthe magnetic flux producing means of said discharge magnetic brushmeans, said magnetic flux producing means of said discharge magneticbrush means being positioned to attract developer'from said feedmagnetic brush means to the cylinder of said discharge magnetic brushmeans and hold said developer on said cylinder as it rotates saiddeveloper into contact with said insulating surface and then returns itto said reservoir, means for rotating the cylinders of said magneticbrush means at predetermined peripheral rates said predetermined ratesbeing so chosen relative to the movement of said insulating surface tocreate a substantial accumulation of tumbling developer in said cavity.15. Apparatus according to claim 24 wherein the cylinders of saidmagnetic brush means are positioned so that at their closest points theouter surface of the cylinder of said feed magnetic brush means isfarther from the insulating surface than the outer surface of thecylinder of the discharge magnetic brush-means.

16. Apparatus according to claim 14 wherein said means for rotating thecylinders includes means to rotate the cylinder of said feed brush meansat a faster peripheral speed than the peripheral speed of the cylinderof said discharge brush means.

17. Apparatus for developing an electrostatic image carried on a movinginsulating surface comprising:

a trough for magnetic brush developer powder;

a first and second magnetic brush operatively located with respect tosaid trough, each brush having a rotatable nonmagnetic cylinder;

a series of stationary magnetic poles accurately spaced in cylindricalformation inside each cylinder and spaced from said cylinder;

portions of said first and second magnetic brushes being positionedrelative to said moving'insulating surface to define a rollback cavitytherewith; and means for rotating the cylinders of said magnetic brushesin Disclaimer 3,543,72O.-Roge1- A. Dremler and Conrad Altmann,Rochester, N. Y. APPARA- TUS FOR DEVELOPMENT OF ELECTROSTATIC IMAGES.Patent dated Dec. 1, 1970. Disclaimer filed Jan. 23, 1973, by theassignee, Eastman K odak Company. Hereby enters this disclaimer toclaims 1 through 13 of said patent.

[Official Gazette May 22, 1.973.]

